Creating a flow standard for harsh environmental conditions

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How ambient air monitoring technicians helped craft the FP-25

Breathe in the crisp, frigid air. Feel the snow blowing against your face. Such weather would push most flow standards well beyond their limits of accuracy. Not so for the FP-25. It was tailor-made for these very conditions, thanks to technicians just like you.

Alicat FP-25 flow calibrating a Thermo Scientific Partisol 2000 FRM air sampler

Alicat FP-25 flow calibrating a Thermo Scientific Partisol 2000 FRM air sampler

This is the story of how ambient air monitoring technicians helped craft the FP-25, Alicat’s new weatherproof flow standard for outdoor air samplers. This instrument is the product of an iterative process that was centered upon continuous user feedback. We simply built what air monitoring technicians told us what they wanted, plus a little bit more.

The story begins with air monitoring technicians who were already using Alicat’s portable mass flow meters inside their shelters. These technicians had become frustrated with the lengthy 20-point calibration process for the mass flow controllers inside their gas dilution calibrators. Replacing their positive displacement meters with Alicats reduced their flow calibration times at each point to just a few seconds, even at 5 sccm.

"Whisper" portable mass flow calibrator, shown with optional color display

“Whisper” portable mass flow calibrator

After Alicat released the Whisper series of low pressure drop flow meters, some of these technicians began to apply this new technology to calibrating their outdoor ambient air samplers. Outdoor flow measurement is no easy task for a high-precision, laboratory-based flow standard. So, we got to work figuring out how to maintain our laboratory-grade accuracy under the harsh environmental conditions that these technicians faced.

Reducing temperature equilibration time

Alicat FP-25 flow calibrating a Thermo Scientific 2025i FRM air sampler

Alicat FP-25 flow calibrating a Thermo Scientific 2025i FRM air sampler

Technicians who used our lab-based Whisper flow meters told us that they took a long time to equilibrate to ambient temperatures outside, especially on sunny days. Big blocks of stainless steel with embedded temperature sensors worked well in the lab, but not so much outdoors. We switched out the steel for aluminum and reduced the size of the block, but we still had to contend with the issue of radiated heat.

The solution to reducing equilibration time started with lowering the temperature sensor deeper into the flow path. This made it more accurate in measuring the precise air temperature, even if was artifically elevated due to radiant heating by the sun. Then we added a fast-responding external temperature probe that the FP-25 uses to correct the internal flow stream temperature to ambient conditions. Together, this setup means that you can start using your flow standard more quickly and keep using it under direct sunlight on a hot day. Even coming out of a heated vehicle into sub-zero ambient temperature requires just a few minutes for equilibration.

Maintaining flow accuracy below zero

Air monitoring happens year-round, so technicians in the northern US and Canada perform outdoor flow calibrations at temperatures as low as -30°C. These subzero conditions wreak havoc on electronic instruments, reducing battery life, obscuring LCD screens and making o-rings brittle. At temperatures below freezing, the pistons of positive displacement meters freeze up, rendering them unusable. In the case of Venturis, their pressure sensors become less responsive, which makes their response less predictable, and so they become less accurate.

The problem of maintaining accuracy below zero was a tough one. Electronic pressure sensors tend to undergo changes at subzero temperatures that result in altered electrical responses to the physical deformations of the sensor. Alicat’s engineers devised a creative solution to this well-understood problem. We subject the pressure sensors of every FP-25 to the full range of operational temperature, from -30°C to +60°C, monitoring their response at multiple points along the way. The resulting response curve is unique to each instrument and becomes part of its final flow and pressure calibration. On account of this advanced process, our FP-25 has the most accurate flow calibration available on an Alicat instrument. At any temperature within its wide operating range, your FP-25 will be accurate to better than 1% of reading.

Combating wind and dust…and snowflakes

Alicat FP-25 flow calibrating a Met One BAM 1020/1022 FEM air sampler

Alicat FP-25 flow calibrating a Met One BAM 1020/1022 FEM air sampler

Some air monitoring technicians have reported that wind can be a significant problem during flow calibrations. They noted that even a snowflake can cause their Venturis to choke. Because volumetric flow standards typically feature minimal pressure drop, so as not to influence the volumetric flow readings, they are also quite sensitive to wind. Indeed, the Alicat Whisper flow meter that is sitting on my desk as I write this reveals the presence of airflow when my office air conditioner kicks on. Although this sensitivity makes the instrument more accurate, it can also make flow measurement more difficult when the wind blows. Windy days can also send dust, or snowflakes, through your flow standard.

To resolve this issue, we took a cue from the manufacturers of air samplers, who long ago figured out the solution to wind and dust. Air sampler inlets incorporate screens to filter out large dust particles, and their louvered designs prevent windy conditions from influencing the flow rate within their downtubes. The Alicat FP-25 takes full advantage of these features of the sampler inlet when it is mounted in line with the air sampler’s downtube, a configuration we call Direct Mode. In this configuration, the FP-25 is positioned immediately below the PM10 inlet, but above the PM2.5 very sharp cut cyclone (VSCC), if used. This affords the flow sensor full protection from wind, dust, rain and snowflakes, and it enables the FP-25 to monitor the very same flow conditions that the sampler sees under normal operation.

Your late winter forecast may call for heavy snow or blustery, sunny days. If you’re an air monitoring technician, the FP-25 has you covered in either case. If you’re in another industry that has you measuring flow in adverse environmental conditions, let us know. We’re eager to help.

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